From 9e6bdef224f700c057462a7d5e9b4a2770e04569 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Marc-Andr=C3=A9=20Lureau?= Date: Fri, 31 Aug 2018 16:53:12 +0200 Subject: util: add qemu_write_pidfile() MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit There are variants of qemu_create_pidfile() in qemu-pr-helper and qemu-ga. Let's have a common implementation in libqemuutil. The code is initially based from pr-helper write_pidfile(), with various improvements and suggestions from Daniel Berrangé: QEMU will leave the pidfile existing on disk when it exits which initially made me think it avoids the deletion race. The app managing QEMU, however, may well delete the pidfile after it has seen QEMU exit, and even if the app locks the pidfile before deleting it, there is still a race. eg consider the following sequence QEMU 1 libvirtd QEMU 2 1. lock(pidfile) 2. exit() 3. open(pidfile) 4. lock(pidfile) 5. open(pidfile) 6. unlink(pidfile) 7. close(pidfile) 8. lock(pidfile) IOW, at step 8 the new QEMU has successfully acquired the lock, but the pidfile no longer exists on disk because it was deleted after the original QEMU exited. While we could just say no external app should ever delete the pidfile, I don't think that is satisfactory as people don't read docs, and admins don't like stale pidfiles being left around on disk. To make this robust, I think we might want to copy libvirt's approach to pidfile acquisition which runs in a loop and checks that the file on disk /after/ acquiring the lock matches the file that was locked. Then we could in fact safely let QEMU delete its own pidfiles on clean exit.. Signed-off-by: Marc-André Lureau Message-Id: <20180831145314.14736-2-marcandre.lureau@redhat.com> Signed-off-by: Paolo Bonzini --- util/oslib-posix.c | 68 ++++++++++++++++++++++++++++++++++++++++++++++++++++++ util/oslib-win32.c | 27 ++++++++++++++++++++++ 2 files changed, 95 insertions(+) (limited to 'util') diff --git a/util/oslib-posix.c b/util/oslib-posix.c index 13b6f8d776..0e3ab9d959 100644 --- a/util/oslib-posix.c +++ b/util/oslib-posix.c @@ -88,6 +88,74 @@ int qemu_daemon(int nochdir, int noclose) return daemon(nochdir, noclose); } +bool qemu_write_pidfile(const char *path, Error **errp) +{ + int fd; + char pidstr[32]; + + while (1) { + struct stat a, b; + + fd = qemu_open(path, O_CREAT | O_WRONLY, S_IRUSR | S_IWUSR); + if (fd == -1) { + error_setg_errno(errp, errno, "Cannot open pid file"); + return false; + } + + if (fstat(fd, &b) < 0) { + error_setg_errno(errp, errno, "Cannot stat file"); + goto fail_close; + } + + if (lockf(fd, F_TLOCK, 0) < 0) { + error_setg_errno(errp, errno, "Cannot lock pid file"); + goto fail_close; + } + + /* + * Now make sure the path we locked is the same one that now + * exists on the filesystem. + */ + if (stat(path, &a) < 0) { + /* + * PID file disappeared, someone else must be racing with + * us, so try again. + */ + close(fd); + continue; + } + + if (a.st_ino == b.st_ino) { + break; + } + + /* + * PID file was recreated, someone else must be racing with + * us, so try again. + */ + close(fd); + } + + if (ftruncate(fd, 0) < 0) { + error_setg_errno(errp, errno, "Failed to truncate pid file"); + goto fail_unlink; + } + + snprintf(pidstr, sizeof(pidstr), FMT_pid "\n", getpid()); + if (write(fd, pidstr, strlen(pidstr)) != strlen(pidstr)) { + error_setg(errp, "Failed to write pid file"); + goto fail_unlink; + } + + return true; + +fail_unlink: + unlink(path); +fail_close: + close(fd); + return false; +} + void *qemu_oom_check(void *ptr) { if (ptr == NULL) { diff --git a/util/oslib-win32.c b/util/oslib-win32.c index 25dd1595ad..b4c17f5dfa 100644 --- a/util/oslib-win32.c +++ b/util/oslib-win32.c @@ -776,3 +776,30 @@ ssize_t qemu_recvfrom_wrap(int sockfd, void *buf, size_t len, int flags, } return ret; } + +bool qemu_write_pidfile(const char *filename, Error **errp) +{ + char buffer[128]; + int len; + HANDLE file; + OVERLAPPED overlap; + BOOL ret; + memset(&overlap, 0, sizeof(overlap)); + + file = CreateFile(filename, GENERIC_WRITE, FILE_SHARE_READ, NULL, + OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL); + + if (file == INVALID_HANDLE_VALUE) { + error_setg(errp, "Failed to create PID file"); + return false; + } + len = snprintf(buffer, sizeof(buffer), FMT_pid "\n", (pid_t)getpid()); + ret = WriteFile(file, (LPCVOID)buffer, (DWORD)len, + NULL, &overlap); + CloseHandle(file); + if (ret == 0) { + error_setg(errp, "Failed to write PID file"); + return false; + } + return true; +} -- cgit v1.2.3